The disclosure of Japanese Patent Application No. 11-362735 filed on Dec. 21, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an evaporative emission control system and, more particularly, to an evaporative emission control system for a fuel tank installed in a motor vehicle, such as an automobile.
2. Description of Related Art
It has been known in the art that a fuel tank installed in a motor vehicle, such as an automobile, be equipped with an evaporative emission control system which prevents fuel evaporative emission (fuel vapors) generated in the fuel tank from flowing or escaping to the outside of the tank. One example of such a system is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. HEI 8-276757.
In such an evaporative emission control system, as shown in FIG. 10 by way of example, a back pressure chamber 74 and a tank chamber 76 of a differential pressure valve 72 mounted in a fuel tank 70 are separated or partitioned from each other by a diaphragm, constituting a valve portion 80. A communicating hole 82 for communication between the back pressure chamber 74 and the tank chamber 76 of the differential pressure valve 72 is formed through the valve portion 80. With this arrangement, liquid fuel that has flown into he back pressure chamber 74 of the differential pressure valve 72 may be returned into the fuel tank 70 through the communicating hole 82.
In the above evaporative emission control system, if the liquid surface 84A of the fuel 84 in the fuel tank 70 is inclined such that its level becomes higher on the side of a fuel fill pipe 86 than on the other side of the tank, during a vehicle turn or other mode of running of the vehicle, as shown in FIG. 10, the air pressure in the back pressure chamber 74 escapes or releases into the tank chamber 76 through the communicating hole 82, as indicated by an arrow W1 in FIG. 10. Thus, a large amount of fuel 84 is likely to flow into the fuel fill pipe 86 and a recirculation conduit 88 formed in communication with the fill pipe 86.
When the fuel tank 70 shifts from the state as shown in FIG. 10, to the state in which the liquid surface 84A of the fuel 84 in the fuel tank 70 is inclined such that its level is lower on the side of the fuel fill pipe 86 than on the other side as shown in FIG. 11, a relatively long period of time is required until a large amount of fuel 84 that has entered the fill pipe 86 and the recirculation conduit 88 returns to the inside of the fuel tank 70. During the period of time in which the large amount of fuel 84 that has entered the fill pipe 86 and the recirculation conduit 88 takes to return to the inside of the fuel tank 70, a negative pressure builds up in the back pressure chamber 74 of the differential pressure valve 72, whereby the valve portion 80 is placed in the open position. As a result, a large amount of evaporative fuel flows from the fuel tank 70 to a canister (not shown).
It is therefore an object of the present invention to provide an evaporative emission control system capable of reducing the amount of evaporative fuel that flows into a canister during running of the vehicle.
To accomplish the above object, the present invention provides an evaporative emission control system. A differential pressure valve controls fluid communication between a canister and a fuel tank. The differential pressure valve includes a diaphragm that separates a first chamber and a second chamber of the differential pressure valve from each other, and a valve seat that faces the diaphragm, the differential pressure valve being placed in a selected one of an open state in which the diaphragm is spaced apart from the valve seat, and a closed state in which the diaphragm rests on the valve seat. The diaphragm has a communicating hole formed therethrough. A closure member is positioned and configured to close the communicating hole of the diaphragm when the differential pressure valve is placed in the closed state, the communicating hole communicating with the first and second chambers when the differential pressure valve is placed in the open state.
In the aforementioned evaporative emission control system, when the liquid surface of fuel within the fuel tank is inclined during a vehicle turn or other running mode of the vehicle, such that the level of the liquid surface becomes higher on the side of a fuel fill pipe than on the other side of the fuel tank, the differential pressure valve is placed in the closed state with the diaphragm resting upon the valve seat, and therefore the communicating hole formed through the diaphragm is closed by the closure member. As a result, the air pressure in the back pressure chamber is inhibited from escaping into the tank chamber through the communicating hole, and therefore only a small amount of fuel is introduced into the fuel fill pipe. If the liquid surface of fuel within the fuel tank is subsequently inclined such that the level of the liquid surface becomes lower on the side of the fuel fill pipe than on the other side of the fuel tank, a negative pressure is produced in the back pressure chamber of the differential pressure valve, whereby the diaphragm is lifted away from the valve seat. In this state, since the communicating hole communicates with the back pressure chamber and the tank chamber, the small amount of fuel that has entered the fill pipe can return to the fuel tank within a short period of time. Although the diaphragm is spaced apart from the valve seat (the differential pressure valve is held in the open state) while the fuel that has entered the fill pipe 12 is returning to the fuel tank, it is possible to restrict or reduce the amount of evaporative fuel flowing toward the canister during running of the vehicle because only a short period of time is required to drain the fill pipe.
To accomplish the above object, the present invention also provides a differential pressure valve that controls fluid communication between a canister and a fuel tank. The differential pressure valve includes a diaphragm that separates a back pressure chamber and a tank chamber of the differential pressure valve from each other, and a valve seat that faces the diaphragm, the differential pressure valve being opened when the diaphragm is moved away from the valve seat. The differential pressure control valve includes a communicating portion that communicates with the back pressure chamber and the tank chamber, and a closure member positioned and configured to close the communicating portion when a pressure within the back pressure chamber is higher than a pressure within the tank chamber. The closure member permits the communicating portion to communicate with the back pressure chamber and the tank chamber before opening of the differential pressure valve when the pressure within the back pressure chamber becomes lower than the pressure within the tank chamber.
In one preferred embodiment of the invention, the closure member as indicated just above may be a check valve.
In the aforementioned evaporative emission control system, when the liquid surface of fuel within the fuel tank is inclined during a vehicle turn or other running mode of the vehicle, such that the level of the liquid surface becomes higher on the side of the fuel fill pipe than on the other side of the fuel tank, a pressure higher than at least the pressure in the tank chamber is applied to the back pressure chamber of the differential pressure valve, and the communicating portion between the back pressure chamber and the tank chamber is closed by the check valve. As a result, the air pressure in the back pressure chamber is kept from escaping into the tank chamber through the communicating portion, and therefore only an extremely small amount of fuel flows into the fuel fill pipe. If the liquid surface of fuel within the fuel tank is subsequently inclined such that the level of the liquid surface becomes lower on the side of the fuel fill pipe, the fuel that has entered the fill pipe can be returned to the fuel tank within an extremely short period of time. With the fuel in the fuel fill pipe thus returned to the fuel tank, a negative pressure builds up in the back pressure chamber of the differential pressure valve, with a result that the check valve is opened before the diaphragm is lifted away from the valve seat. Thus, the fuel that has entered the fill pipe can be returned to the fuel tank without requiring the diaphragm to be lifted away from the valve seat. It is thus possible to prevent or inhibit evaporative fuel from flowing into the canister during running of the vehicle.